Under the auspices of a National Cooperative Natural Product Drug Discovery Group supported by the National Cancer Institute, the potential antitumor activity of approximately 2500 extracts derived from globally collected plants was evaluated in a panel of enzyme based assays and in a battery of cultured human tumor cell lines. One such extract, prepared from the stem bark of Ziziphus mauritiana Lam. (Rhamnaceae), displayed selective cytotoxicity against cultured human melanoma cells (Nature Medicine, Vo. 1 (10), 1995, WO 96/29068). As a result of bioactivity guided fractionation, betulinic acid, a pentacyclic triterpene, was identified as a melanoma-specific cytotoxic agent. In follow-up studies conducted with a thymic mice carrying human melanomas, tumor growth was completely inhibited without toxicity. As judged by a variety of cellular responses, antitumor activity was mediated by the induction of apoptosis.
A number of triterpenoids, including betulinic acid, have several known medical applications, including use as anticancer drugs. Anderson et al., in WO 95/04526, have discussed the derivatives of triterpenoids which have been used in cancer therapy, including their activity against polyamines which are required by cells to grow at an optimal rate. Some of these triterpenoids have been found to interfere with enzymatic synthesis of polyamines required for optimal cell growth, and thus inhibit the growth of cancer cells, particularly by inhibiting ornithine decarboxylase (Yasukawa, K. et al., Oncology 48: 72-76, 1991), The anti-cancer activity of betulinic acid and some derivatives has been demonstrated using mouse sarcoma 180 cells implanted subcutaneously in nude mice (JP 87,301,580). Choi et al have shown that betulinic acid 3-monoacetate, and betulinic acid methyl ester exhibit ED.sub.50 values of 10.5 and 6.8 .mu.g/ml, respectively, against p388 lymphocytic leukemia cells (Choi, Y-H et al., Planta Medical vol. XLVII, pages 511-513, 1988).
Derivatives having ED.sub.50 values greater than 4.0 .mu.g/ml are considered not to have any significant anticancer activity. Table 1 indicates anticancer activity (ED.sub.50 values) of a number of betulinic acid derivatives.